Satellites Give a Heads Up on Thunderstorms

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Thunderstorm predictions are notoriously unreliable, but
satellites could greatly improve these forecasts, suggests one
research team.

Thunderstorms can bring intense rain, hail, lightning and even
tornadoes, but "predicting them a few hours out is one of the
great problems of meteorology," said Chian-Yi Liu of the
University of Wisconsin-Madison, who works with these satellites.

And the consequences can be more serious than a rained-out hike —
even
major storms can be missed, Liu said, such as the one that
dumped up to 10 inches (25 centimeters) of rain on La Crosse,
Wis., on Aug. 18, 2007.

"Predictions for the day said a moderate chance of
thunderstorms," Liu said, "but this one produced an inch or two
of rain per hour and caused severe flooding."

Liu is making his case for using satellite data to improve
thunderstorm forecasts at the 2010 fall meeting of the American
Geophysical Union in San Francisco this week.

Satellite spies

Thunderstorms are called "convective storms" because they are
powered by differences in air density that cause updrafts and
cooling, and can lead to hail, rain, tornadoes and lightning.
[Related:
Report Severe Weather With Twitter.]

At this week's meeting, Liu planned to demonstrate that data from
a satellite could greatly improve the accuracy of thunderstorm
prediction a few hours out.

"Scientists understand the basic causes of thunderstorm
formation," Liu said, "but their major source of data is usually
surface observations, or measurements taken from balloons that
are released into the lower atmosphere, and they usually lack
information about the upper atmosphere."

When Liu and his colleagues introduced data on conditions at
15,000 to 32,000 feet (4,570 to 9,750 meters) of altitude into
the equation, they found a considerable improvement in the
crucial 3-to-6 hour forecast. The data was collected from 400
different events by sensors on NASA's Aqua satellite that measure
conditions at
different altitudes of the atmosphere.

Vexing convection

Convective storms allow the atmosphere to dump excess energy,
held in the form of heat and humidity, and release it as wind
and, especially, precipitation. Convection storms are most likely
to occur when the atmosphere is unstable, Liu said.

"Our analysis shows that if there is instability at around 30,000
feet [9,144 m], with other storm-favorable conditions, a
convective storm will develop in the following three to five
hours. Using the top-down view of a satellite reverses our usual
way of thinking about convective storms, and may suggest an
explanation for storms that arise when they would not be
predicted using conventional methods," Liu said.

"For a long time, we have looked at convection and instability
from a near-surface perspective," says co-author Steve Ackerman,
a professor of meteorology and director of CIMSS. "What Chian-Yi
has showed is that this is not always the case, you can drive
instability from the upper troposphere, too."

The troposphere is the lowest layer of the atmosphere, with a
thickness of about 6 miles (less than10 kilometers).

Convection releases energy and feeds on itself, Ackerman said.
"If you have unstable conditions in the atmosphere and things get
moving, they will continue to move by themselves. Our perspective
has been how it could get started from the ground. Chian-Yi has
shown that it can start from the top as well."